A SKIN DISEASE OF FISHES 269 



fish, more especially fresh-water fish, indiscrimin- 

 ately, but is more especially associated with Cyp- 

 rinoids, such as carp, the Salmonidae, pike and 

 perch, and flourishes equally well in any of them. 



Most of the organisms considered heretofore have 

 lived in the blood or in the gut of their hosts. 

 Ichthyophthirius is a skin parasite, and as far as is 

 known, lives in the skin and the layers immediately 

 beneath it only. It attains a relatively large size, 

 some being J mm. to mm. in length. The parasites 

 are somewhat pear-shaped when they are young 

 (Fig. 52, A), but as they get older, they become 

 more spherical. Myonemes are present on the body. 

 The Ciliate possesses a rounded mouth surrounded 

 by a thick lip, and this leads to a short cell gullet. 

 The body contains a number of small, contractile 

 vacuoles scattered in it. Cilia are present, but they 

 are fine and delicate. The macronucleus (Fig. 52, B) 

 is conspicuous, and in the trophic period of life is 

 horseshoe-shaped, and lies in the centre of the body. 

 The micronucleus is small and distinct in the young 

 animal (Fig. 52, A), but as growth proceeds, the 

 micronucleus seems to disappear, and its actual fate 

 is not known with certainty. 



The presence of Ichthyophthirius can be determined 

 by the presence of numerous small whitish pustules 

 on the skin, and amongst them small patches from 

 which scales are absent and which sometimes are 

 bleeding, owing to the breaking away of the parasite. 

 The pustules are most common on the head and fins, 

 but are also found on the eyes and gills, and in the 

 latter situation they produce bleeding. In a very 



